A number of techniques and apparatus are known for characterising the properties of the formation surrounding boreholes. Typically a tool body, comprising a signal source and sensors, is placed in the borehole and the source used to interrogate the formation. A signal returning to the tool body is measured and the parameter of interest calculated. WO96/08733 and U.S. Pat. No. 5,608,215 disclose neutron-neutron and neutron-gamma techniques for determining the density of a formation.
One well known suite of measurements is known as the ‘triple combo’ and is implemented using a measurement tool capable of measuring formation resistivity, porosity and density (and natural gamma ray) properties, and potentially several ancillary measurements.
Typical triple combo measurements arrange the devices for taking measurements in a vertical sequence along the tool body. In many cases a resistivity measurement device (induction, propagation, laterolog etc) is located at the bottom of the tool string, with a neutron measurement device following above. A natural gamma ray detector of the tool is located in such a way that it is not influenced by activation from a leading neutron source. One example of a known wireline triple combo tool is from Schlumberger and comprises a Dual Induction Tool (DIT) measuring resistivity, a Litho Density Tool (LDT) measuring gamma ray density, and a Compensated Neutron Tool (CNT) measuring porosity.
Locating the different sections down the tool increases the length of the tool. Reducing the length of the tool reduces the distance between measurements which helps in the interpretation of results and also reduces the time for rig up and rig down, and can help reduce the risk of the tool getting stuck in the wellbore. One example of a shorter wireline tool with similar capabilities is the Platform Express tool of Schlumberger which utilises integration of the structure of the various functional sections to achieve shorter overall length. Platform Express tools comprise an Array Induction Tool (AIT), a Three Detector Lithology Density tool (TLD), a Highly Integrated Gamma Ray Neutron Sonde (HGNS). A Micro Cylindrically Focused Log (MCFL) for shallow resistivity is co-located with a High Resolution Resistivity Gamma Detector (HRGD) for density measurement in a pad.
U.S. Pat. No. 7,073,378 describes a tool having a resistivity measurement device comprising a multiplicity of antennae interleaved with a neutron measurement device to reduce the length of the tool.
In logging while drilling (LWD) tools certain measurement sections have been co-located in a single collar, for example, the Compensated Density Neutron (CDN) tool and Azimuthal Density Neutron (ADN) tool of Schlumberger.
The axial separation of the different measurement sections means that the investigation of all the formation properties cannot take place during the same time interval. This means that there can be changes in the formation properties (invasion, damage), the borehole (mud weight, borehole size, rugosity) or the tool position in the borehole (standoff) between the multiple measurements.
This invention aims to provide techniques for locating the various measurements closer together such that the interpretation of the measurements can be simpler and more accurate.